Transportation's Role in Reducing U.S. Greenhouse Gas Emissions ...

Transportations Role in Reducing U.S. Greenhouse Gas Emissions: Volume 2
Enhanced Propeller Design
Overview
Conventional fixed-pitch propellers are most efficient at one rotational speed and load
condition, with the propeller utilizing the maximum amount of power from the engine.
At any other rotational speed or operating load, a
fixed-pitch propeller is either “over-pitched” or
“under-pitched,” which leads to a reduction in fuel
efficiency and an increase in emissions. Controllablepitch
propellers have been designed to optimize
efficiency for any speed and load condition. As with
diesel/electric configurations, the fuel and emission
benefits associated with controllable-pitch propellers
are most apparent for vessels that frequently change
speed or load such as harbor vessels, cruise ships,
ferries, and roll-on/roll-off vessels.
Counter-rotating propellers include a pair of
propellers positioned along the same axis as the Figure 3.10 Counter-Rotating
standard propeller, as shown in Figure 3.10 (Ämmälä, Propellers
2006). Counter-rotating propellers can be configured
as independent propellers or can be coupled together, but have independent drive shafts
that rotate in opposite directions. In this configuration the rear propeller recovers
rotational energy from the front propeller, which is used to generate power for the
vessel’s electrical system (Frey and Kuo, 2007).
To obtain the maximum thrust for propulsion, a
propeller must quickly move large volumes of
water. Friction losses occur at the tip of each blade
as water escapes from the high pressure to the lowpressure
side of the blade, reducing the effectiveness
of the propeller. Large nozzles that enclose the
propeller (as shown in Figure 3.11) reduce friction
loses by restricting water flow to the propeller tips.
At the entrance of the nozzle the diameter is greater
than at the trailing throat. This design forces the
water to accelerate from the front of the nozzle to
the rear, increasing the speed of the water as it
reaches the propeller, and allowing the propeller to
move more water and create more thrust for the
same input power and torque (Rice, 2009).
Figure 3.11 Nozzles Enclosing
Propeller to Reduce Friction Losses
Another approach to reducing friction loss at the tips of propeller blades involves the
application of tip winglets (Lumin, 2005; Technical University of Denmark, 2006;
European Commission, 2002). This approach is similar to the wing tip devices used for
aviation that prevent vortex formation. This technology was initially used in recreational
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